The CernVM File System (CernVM-FS) is a read-only file system designed to deliver scientific software onto virtual machines and physical worker nodes in a fast, scalable, and reliable way. Files and file metadata are downloaded on demand and aggressively cached. For the distribution of files, CernVM-FS uses a standard HTTP [BernersLee96] [Fielding99] transport, which allows exploitation of a variety of web caches, including commercial content delivery networks. CernVM-FS ensures data authenticity and integrity over these possibly untrusted caches and connections. The CernVM-FS software comprises client-side software to mount “CernVM-FS repositories” (similar to AFS volumes) as well as a server-side toolkit to create such distributable CernVM-FS repositories.

General overview over CernVM-File System's Architecture

A CernVM-FS client provides a virtual file system that loads data only on access. In this example, all releases of a sofware package (such as an HEP experiment framework) are hosted as a CernVM-FS repository on a web server.

The first implementation of CernVM-FS was based on grow-fs [Compostella10] [Thain05], which was originally provided as one of the private file system options available in Parrot. Ever since the design evolved and diverged, taking into account the works on HTTP- Fuse [Suzaki06] and content-delivery networks [Freedman03] [Nygren10] [Tolia03]. Its current implementation provides the following key features:

  • Use of the the Fuse kernel module that comes with in-kernel caching of file attributes
  • Cache quota management
  • Use of a content addressable storage format resulting in immutable files and automatic file de-duplication
  • Possibility to split a directory hierarchy into sub catalogs at user-defined levels
  • Automatic updates of file catalogs controlled by a time to live stored inside file catalogs
  • Digitally signed repositories
  • Transparent file compression/decompression and transparent file chunking
  • Capability to work in offline mode providing that all required files are cached
  • File system versioning
  • File system hotpatching
  • Dynamic expansion of environment variables embedded in symbolic links
  • Support for extended attributes, such as file capabilities and SElinux attributes
  • Automatic mirror server selection based on geographic proximity
  • Automatic load-balancing of proxy servers
  • Support for WPAD/PAC auto-configuration of proxy servers
  • Efficient replication of repositories
  • Possibility to use S3 compatible storage instead of a file system as repository storage

In contrast to general purpose network file systems such as nfs or afs, CernVM-FS is particularly crafted for fast and scalable software distribution. Running and compiling software is a use case general purpose distributed file systems are not optimized for. In contrast to virtual machine images or Docker images, software installed in CernVM-FS does not need to be further packaged. Instead it is distributed and versioned file-by-file. In order to create and update a CernVM-FS repository, a distinguished machine, the so-called Release Manager Machine, is used. On such a release manager machine, a CernVM-FS repository is mounted in read/write mode by means of a union file system [Wright04]. The union file system overlays the CernVM-FS read-only mount point by a writable scratch area. The CernVM-FS server tool kit merges changes written to the scratch area into the CernVM-FS repository. Merging and publishing changes can be triggered at user-defined points in time; it is an atomic operation. As such, a CernVM-FS repository is similar to a repository in the sense of a versioning system.

On the client, only data and metadata of the software releases that are actually used are downloaded and cached.

CernVM-FS client architectural overview

Opening a file on CernVM-FS. CernVM-FS resolves the name by means of an SQLite catalog. Downloaded files are verified against the cryptographic hash of the corresponding catalog entry. The stat() system call can be entirely served from the in-kernel file system buffers.